The danger of natural gas leakage is well documented. But monitoring of leakage of natural gas at a customer-end user's home, business and the like, except by using an odorant, is of low priority. Reason: dangers due to low concentrations of natural gas is not well-documented. However, as the age of the customer's piping networks increase, the frequency of low concentration leakage become large enough to be a sufficient threat to the health and safety of the user.
Also, in my copending application, Ser. No. 07/926,457 filed Aug. 8, 1992 for "METHOD AND MEANS FOR FILTERING ODORANT FROM A GAS STREAM", I teach that odorant such as mercaptan are sufficient threat to health to warrant their removal by the user within his own internal piping network and in the above application, are removed from the natural gas flow within a customer's domain by filtering. However, while newer customer piping systems may be sufficient to not require direct and active monitoring even if the odorant is filtered from the natural gas stream as I propose, older internal piping systems may require an active method of leakage detection under such circumstances. While EPA and various State Agencies may be aware of the problem of leakage in a customer's internal gas lines, they do not think the reported exposure is of sufficient levels or a large enough frequency, to be dangerous.
In such situation, I find that surprisingly large concentrations of leakage occur in older internal piping systems. Such systems are historically constructed of metal materials resulting in conventional black iron, galvanized and copper piping networks. I have found that an average internal piping network of a customer is 40-60 feet in length and comprises numerous fittings and lengths of piping attached together, say in the range of about 40 to 90 connections. With time, such joints can leak due to various conditions, such as corrosive nature of natural gas, temperature variation, mechanical stress due to aging of supporting structure and seismic ground movement.
The American Gas Association recommends minimum surveys--once a year in business districts and every 5 years outside business areas. These surveys are made by one of the following methods: manhole and bar hole surveys using combustible gas indicators, vegetation, pressure drop and visual surveys and mobile infra-red surveys. However, within the customer's domain, only inaccurate and clumsy methods are available. For example, one method uses the gas meter itself. Its dial is checked under FLOW-NO FLOW conditions. Then an appliance is turned on and the meter re-checked. If the meter dial does not move, the meter is defective.
Another method uses a manometer attached to an appliance outlet. After calibration, FLOW AND NO-FLOW conditions are observed. If the latter conditions result in a pressure drop reading, all outlets in the stream should be examined for leakage.
However, at present there is no on-line method to detect gas leakage within the end user's internal piping network.